Nukleofilens

Nucleophiles are chemical species that donate an electron pair to an electrophilic center in a reaction. They are typically electron-rich due to negative charge or lone pairs, and they seek positively charged or partially positive carbon atoms or other electrophilic sites. Common nucleophiles include halide ions (such as Cl−, CN−), alkoxide and amide bases (RO−, NR2−), cyanide, water, alcohols, amines, enolates, and thiolates. The strength of a nucleophile depends on its charge, the availability of its electron pair, and its polarizability.

Nucleophiles are classified by their hardness or softness, a concept captured by HSAB theory. Hard nucleophiles (small, highly charged species) tend to attack hard electrophiles like carbonyl carbons in simple aldehydes and ketones, whereas soft nucleophiles (more polarizable) favor softer electrophiles such as allylic or benzylic halides. Nucleophilicity also correlates with solvent: in polar aprotic solvents, many anions become highly reactive nucleophiles, while protic solvents hydrogen-bond and solvate ions, reducing their nucleophilicity.

Reaction mechanisms involving nucleophiles include bimolecular nucleophilic substitution (SN2), where the nucleophile attacks the substrate simultaneously with the leaving group departure, and unimolecular pathways (SN1) where a carbocation forms first. Nucleophiles also participate in additions to carbonyl compounds (nucleophilic addition to aldehydes and ketones) and conjugate (Michael) additions to α,β-unsaturated systems.

In synthesis, nucleophiles enable the formation of C–C and C–heteroatom bonds, influence stereochemistry, and are shaped by solvent, temperature, and the nature of the electrophile.